Integrated circuit structure with vertical isolation from single crystal substrate comprising isolation layer formed by implantation and annealing of noble gas atoms in substrate

ABSTRACT

An integrated circuit structure vertically isolated electrically from the underlying substrate is formed in/on a single crystal semiconductor substrate, such as a silicon semiconductor wafer, by first implanting the substrate with a sufficient dosage of noble gas atoms to inhibit subsequent recrystallization of the semiconductor lattice in the implanted region during subsequent annealing, resulting in the formation of an isolation layer comprising implanted noble gas atoms enmeshed with semiconductor atoms in the substrate which has sufficient resistivity to act as an isolation layer. The preferred noble gases used to form such isolation layers are neon, argon, krypton, and xenon. When neon atoms are implanted, the minimum dosage should be at least about 6×10 15  neon atoms/cm 2  to inhibit subsequent recrystallization of the silicon substrate. When argon atoms are implanted, the minimum dosage should be at least about 2×10 15  argon atoms/cm 2 . When krypton is implanted, the minimum dosage should be at least about 6×10 14  krypton atoms/cm 2 . The energy used for the implant should be sufficient to provide an average implant depth sufficient to form, after annealing, the noble gas isolation layer at a depth of at least about 0.5 microns from the surface.

This application is a continuation of application Ser. No. 08/461,413,filed Jun. 5, 1995 now abandoned as a division of application Ser. No.08/198,911, filed Feb. 17, 1994 now U.S. Pat. No. 5,508,211.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an integrated circuit structure isolated fromportions of a single crystal substrate beneath the integrated circuitstructure by an isolation layer of noble gas atoms in the substrate.More particularly, this invention relates to an integrated circuitstructure isolated from portions of a single crystal substrate beneaththe integrated circuit structure by an isolation layer of noble gasatoms implanted beneath the surface of the substrate and then annealedto form the isolation layer and a method of making such an isolationlayer.

2. Description of the Related Art

In the formation of a plurality of integrated circuit structures on/in acommon single crystal substrate, e.g., a semiconductor wafer, eachintegrated circuit structure, e.g., active device, is electricallyisolated from other such devices formed on/in the same substrate. Suchelectrical isolation usually includes lateral isolation whichconventionally comprises field oxide or oxide-filled trenches. However,in at least some circumstances, it is also important to provide verticalisolation of the integrated circuit structure from the bulk of thesubstrate beneath the integrated circuit structure, to avoid couplingthrough the bulk of the substrate to other devices formed in/on thesubstrate, to avoid deep punch-through, e.g., by alpha particles, forsoft-error reduction, for depletion containment, and to reduce thedefect density.

Such vertical isolation can be formed by forming an oxide layer beneaththe surface of a substrate by implanting oxygen atoms and then annealingthe implanted oxygen atoms to form the desired isolation. Such astructure is usually referred to as silicon-on-insulator isolation.However, while the formation of such an oxide isolation layer beneaththe surface of the substrate can result in the reduction or eliminationof the aforesaid problems, it is not without problems of its own,including the formation of an electrically floating substrate on/inwhich the integrated circuit structure will be constructed. This is notdesirable because floating substrates are susceptible to bipolarbreakdown, reduced noise immunity, and uncertain transistor performance.

Formation of an oxide isolation layer beneath the surface of a substrateis undesirable because the oxygen implant would have to be performed ata high temperature of about 600° C. at very high doses, e.g., 10¹⁸atoms/cm². No resist would protect it, so the whole wafer would need tobe implanted. At the present time there are only a limited number ofimplanters in existence capable of carrying out such an oxygen implant.

In previous studies of implant dosages of noble gases such as neon,argon, and krypton, into single crystal silicon, the blocking orprevention of epitaxial regrowth of the damaged silicon afterimplantation by some threshold dosage concentration of the noble gasatoms has been discussed. Cullis, Seidel, and Meek, in an articleentitled "Comparative Study of Annealed Neon-, Argon-, and Krypton-IonImplantation Damage in Silicon", published in the Journal of AppliedPhysics, 49 (10), October 1978, at pages 5188-5198, discussedimplantation damage and noted that for dosages of 6×10¹⁵ Ne⁺ atoms percm², 2×10¹⁵ Ar⁺ atoms per cm², or 6×10¹⁴ Kr⁺ atoms per cm², each gave acontinuous disordered zone from the Si surface to the end of the rangeafter initial implantation, while polycrystalline layers were formedupon annealing at 1100° C. for 30 minutes.

Revesz, Wittmer, Roth, and Mayer, in an article entitled "EpitaxialRegrowth of Ar-Implanted Amorphous Silicon" published in the Journal ofApplied Physics, 49 (10), October 1978, at pages 5199-5206, report onthe epitaxial regrowth of silicon after implantation of Argon atoms atdosage levels of 2×10¹⁵ Ar atoms/cm² and 6×10¹⁵ AR atoms/cm². They statethat initially, the regrowth rate is rather high, but slows down withlonger anneals and stops completely after a certain annealing time. Theystated that from these facts one might readily conclude that theregrowth is governed by the Ar concentration and that the regrowth stopscompletely if this concentration reaches a certain amount at theamorphous-crystalline interface.

The same authors (Wittmer, Roth, Revesz, and Mayer) also published apaper entitled "Epitaxial Regrowth of Ne- and Kr-Implanted AmorphousSilicon" in the Journal of Applied Physics, 49 (10), October 1978, atpages 5207-5212. In this paper they discussed the effect of the presenceof such noble gas atoms on bubble growth in the substrate and speculatedthat epitaxial regrowth can be blocked for the Ne- and Kr-implantedcase, as in the Ar-implanted silicon, if the concentration of theimplanted specie exceeds a certain critical value.

Aronowitz, in a paper entitled "Quantum-Chemical Modeling of Boron andNoble Gas Dopants in Silicon", published in the Journal of AppliedPhysics, 54 (7), July 1983, at pages 3930-3934, noted the earlier papersand then calculated that the implanted noble gas atoms would beenergetically less stably constrained within the silicon lattice than iffree, i.e., not within a lattice structure.

While the presence of noble gas atoms in a silicon substrate is,therefore, not unknown, the previous studies of such centered about thepresence of such noble gas atoms in connection with riopant implants, orthe presence of such noble gases in gettering, ion etching, andsputtering processes. That is, the presence of such noble gas atoms inthe silicon substrate was usually ancillary to some other process orreaction and was, therefore studied either as a tolerated impurity or atleast as present or functioning as a supplement to another substance,e.g., a dopant, also present in the substrate.

SUMMARY OF THE INVENTION

Quite surprisingly, therefore, we have discovered that an integratedcircuit structure vertically isolated electrically from the underlyingsubstrate can be formed in/on a single crystal substrate, such as asilicon semiconductor wafer, by first implanting the substrate with asufficient dosage of noble gas atoms to inhibit subsequentrecrystallization during annealing to form an isolation layer of saidnoble gas atoms in the substrate which has sufficient resistivity to actas an isolation layer. The preferred noble gases used to form suchisolation layers are neon, argon, and krypton. When neon atoms areimplanted, the minimum dosage should be at least about 6×10¹⁵ neonatoms/cm² to inhibit subsequent recrystallization of the siliconsubstrate. When argon atoms are implanted, the minimum dosage should beat least about 2×10¹⁵ argon atoms/cm². When krypton is implanted, theminimum dosage should be at least about 6×10¹⁴ krypton atoms/cm². Theenergy used for the implant should be sufficient to provide an averageimplant depth sufficient to form, after annealing, the noble gasisolation layer at a depth of at least about 0.5 microns from thesurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross-sectional view of a single crystalsubstrate being implanted with noble gas atoms.

FIG. 2 is a fragmentary cross-sectional view of the implanted singlecrystal substrate of FIG. 1 after annealing to form the desired noblegas atom isolation layer beneath the surface of the substrate.

FIG. 3 is a fragmentary cross-sectional view of an active devicecomprising a portion of an integrated circuit structure shown formed inthe substrate over the noble gas atom isolation layer shown in FIG. 2.

FIG. 4 is a fragmentary cross-sectional view showing field oxide grownadjacent the integrated circuit structure shown in FIG. 3 to provide, incooperation with the noble gas atom isolation layer, vertical andlateral electrical isolation of the illustrated integrated circuitstructure from other portions of the integrated circuit structure formedon the substrate.

FIG. 5 is a fragmentary cross-sectional view showing an oxide-filledtrench formed adjacent the integrated circuit structure shown in FIG. 3to provide, in cooperation with the noble gas atom isolation layer,vertical and lateral electrical isolation of the illustrated integratedcircuit structure from other portions of the integrated circuitstructure formed on the substrate.

FIG. 6 is a flow sheet illustrating the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises an integrated circuit structure formed in asingle crystal semiconductor substrate and separated from a portion ofthe substrate by a non-oxide isolation layer formed beneath theintegrated circuit structure. The non-oxide isolation layer is formed byimplanting noble gas atoms into the substrate at a dosage levelsufficient to inhibit subsequent recrystallization of the semiconductormaterial upon annealing. The resulting layer of noble gas atoms andsemiconductor atoms has a resistance high enough to provide electricalisolation of the integrated circuit formed thereover from the remainderof the semiconductor substrate beneath the isolation layer, particularlywhen integrated circuit structures capable of operating at 3 volts orless are constructed over the isolation layer.

By use of the term "noble gases" as implant atoms for use herein informing the desired isolation layer is meant neon, argon, krypton, andxenon. While helium is also a noble gas, it has been omitted from thedefinition because it is too light to cause sufficient damage at areasonable dose and the dose needed to create permanent damage would betoo high; and radon has been omitted because of its radioactivity.

Since the purpose of implanting the substrate with these noble gasatoms, followed by annealing, is to form the desired isolation layer inthe semiconductor substrate, it is important that the annealing notresult in subsequent recrystallization of the damaged single crystalsemiconductor layer, with uniform diffusion of the noble gas atomsthroughout the substrate, or at least over the entire implanted volume.By use of the phrase "inhibit subsequent recrystallization" is meantthat the damaged portion of the single crystal semiconductor substrate,i.e., the central portion of the region of the substrate containing theimplanted noble gas atoms, does not recrystallize as single crystalmaterial.

Rather the annealing should result in concentration of the implantednoble gas atoms into a concentrated region of the substrate to form thedesired isolation layer. To do so, it is important that the implantdosage of the noble gas atoms be above a threshold level to providesufficient damage or disruption of the crystal lattice so thatsubsequent annealing will not repair such damage, i.e., will not resultin recrystallization of the disrupted or damaged area. This thresholddosage will vary with the respective noble gas atoms, because of thedifferences in mass between the noble gas atoms.

While we do not wish to be bound by any theories with regard to theformation of the isolation layer, the implanted noble gas atoms tend tomigrate to the area or region of least crystallinity or order.Therefore, the anneal appear to serve the dual purpose ofrecrystallizing the region above the isolation layer as well as causingthe migration or further concentration of the implanted noble gas atomsinto the isolation region where recrystallization is not occurring.Thus, both the implant and anneal steps appear to play a role in theformation of the desired isolation layer where the implanted noble gasatoms are enmeshed with semiconductor atoms, e.g., silicon atoms.

When the noble gas atoms consist of neon atoms, the minimum dosageshould be at least about 6×10¹⁵ neon atoms/cm² at an energy level of atleast 0.75 meV to inhibit recrystallization of the silicon substrate.When argon atoms are implanted, the minimum dosage should be at leastabout 2×10¹⁵ argon atoms/cm² at an energy level of at least 1.5 meV.When krypton atoms are implanted, the minimum dosage should be at leastabout 6×10¹⁴ krypton atoms/cm² at an energy level of at least about 3meV.

It is also important that the isolation layer of noble gas atoms beformed beneath the surface of the semiconductor substrate at asufficient depth to permit the layer of semiconductor above theisolation layer to be of sufficient quality and crystallinity to permitthe formation of the desired integrated circuit structure. To achievethis end, it is necessary to implant the noble gas atoms at a sufficientminimum energy level.

It will be understood that the use of the term "isolation layer" isintended to define a region in the semiconductor substrate where thenoble gas atoms are enmeshed with the semiconductor atoms, e.g., siliconatoms, and the crystallinity of the semiconductor lattice has beensufficiently disrupted by the implant so that recrystallization will notoccur during the subsequent anneal.

The minimum depth, beneath the substrate surface, of the isolation layerto be formed in the single crystal substrate after annealing shouldrange from at least about 0.3 microns (3,000 Angstroms), to about 2microns (20,000 Angstroms), and preferably from about 0.5 microns to 1micron, as measured from the surface down to the uppermost portion ofthe layer. This will ensure the provision of adequate depth of thesubstrate above the isolation layer to permit formation of the desiredintegrated circuit structures on/in the substrate. However, it is alsoimportant that the isolation layer not be so deep in the substrate as todefeat the purpose of the isolation layer, i.e., to isolate theintegrated circuit structure formed thereover from the bulk of thesubstrate. Therefore, the depth of the isolation layer, i.e., thedistance from the surface to the uppermost portion of the isolationlayer, should not exceed about 2 microns, preferably not exceed about1.5 microns, and most preferably not exceed about 1 micron.

In view of the difference in mass of the neon, argon, krypton, and xenonatoms, the minimum energy implant level needed to provide the abovediscussed depth of the resulting isolation layer after annealing willvary with each of the noble gases. For neon, the implant energy toachieve the desired implant depth should range from about 0.75 MeV toabout 1.25 MeV, preferably about 1 MeV. For argon, the implant energyshould range from about 1.5 MeV to about 2.5 MeV, preferably about 2MeV. For krypton, the implant energy should range from about 3 MeV toabout 5 MeV, preferably about 4 MeV. The xenon implant energy shouldexceed the energy used for the krypton implant.

After the implantation, the implanted substrate should be annealed toform the desired isolation layer in the substrate and to recrystallizethe portion of the substrate above the isolation layer. Thisimplantation may be carried out at any conventional annealingtemperature used after implantation for the particular substratematerial. For example, for single crystal silicon substrate material,the substrate may be annealed at a temperature within a range of from900° C. to 1150° C. for a period of from about 1 hour to about 10 hours.The substrate may also be annealed using a rapid thermal annealingtechnique wherein the substrate is rapidly brought up to the annealingtemperature, e.g., within less than 60 seconds, and then held at thistemperature for a period of from about 0.5-3 minutes.

Referring now to the drawings, FIG. 1 shows a single crystal substrate2, such as a single crystal silicon semiconductor wafer, being implantedwith noble gas atoms from a source of such atoms such as an implanternormally used to implant dopants such as boron, phosphorus, or arsenicinto a substrate as is well known to those skilled in the art. After theimplantation, the substrate is annealed as described to form the desiredisolation layer 10 comprising the implanted noble gas atoms, as shown inFIG. 2. formation of isolation layer 10 in substrate 2, an integratedcircuit structure such as, for example, the MOS structure 20 shown inFIG. 3 can be formed in substrate 2 comprising a source region 22 and adrain region 24 formed in substrate 2, gate oxide 26 and a silicon gateelectrode 28 formed over channel portion 30 in substrate 2 betweensource region 22 and drain region 24.

When isolation layer 10 is present in substrate 2, for example, anionized path formed in substrate 2 and extending below isolation layer10, such as formed by impact with radiation such as an alpha particle,as shown at dotted line 100 in FIG. 3, will not result in an ionizedcharge being collected by the sensitive drain region 24.

While isolation layer 10 in FIG. 3 provides isolation of the bulk ofsubstrate 2 from MOS device 20, it is conventional to provide lateraldielectric isolation of such an active device 20 from adjacent portionsof the integrated circuit structure as well. Therefore, as shown in FIG.4, dielectric material such as field oxide (e.g., silicon oxide)portions 40 may be grown in the surface of substrate 2 which preferablyextend down to isolation layer 10 to, therefore, provide completeelectrical isolation of a device such as device 20 from other portionsof substrate 2, including other integrated circuit structures formed insuch other portions.

Similarly, as shown in FIG. 5, lateral electrical isolation of activedevice 20 may also be achieved by forming isolation trench 50 extendingdown from the surface of substrate 2, preferably to intersect isolationlayer 10. Isolation trench 50 may be filled with oxide, e.g., siliconoxide, or other dielectric insulation material, e.g., silicon nitride,or lined with oxide or other dielectric insulation material and thenrefilled with material such as polysilicon.

While a PMOS or p-channel device is shown in FIGS. 3-5, by way ofillustration, it will be readily appreciated that other devices, such asNMOS or bipolar devices may be formed in the substrate and will also beisolated from other regions of the substrate by isolation layer 10.Thus, the invention provides an integrated circuit structure formed in asingle crystal substrate and separated from portions of the substratebeneath the integrated circuit structure by an isolation layer of noblegas atoms formed beneath the surface of the substrate. When combinedwith lateral isolation such as field oxide or isolation trenches formedin the substrate and extending down from the surface of the substrate,an integrated circuit structure may be both vertical and laterallyisolated electrically from other structures formed in the samesubstrate.

Having thus described the invention what is claimed is:
 1. An integratedcircuit structure in a single crystal silicon substrate electricallyisolated from other portions of said silicon substrate by a buriednon-oxide isolation region formed in said silicon substrate beneath saidintegrated circuit structure and spaced from the surface of said siliconsubstrate above said buried non-oxide isolation region, said isolationregion comprising an implant-damaged region of said silicon substratecontaining implanted atoms of a noble gas selected from the groupconsisting of neon, implanted in said substrate at a dosage level of atleast about 6×10¹⁵ neon atoms/cm² and at an energy level ranging fromabout 0.5 MeV to about 1.25 MeV; argon, implanted in said substrate at adosage level of least about 2×10¹⁵ argon atoms/cm² and at an energylevel ranging from about 1.5 MeV to about 2.5 MeV; and krypton,implanted in said substrate at a dosage level of at least about 6×10¹⁴krypton atoms/cm² and at an energy level ranging from about 3 MeV toabout 5 MeV; to inhibit recrystallization during subsequent annealing ofsaid implant-damaged region of said silicon substrate containing saidimplanted atoms of said noble gas, said implanted noble gas atomsenmeshed in said isolation region with silicon atoms from said singlecrystal silicon substrate.
 2. An integrated circuit structure formed ina surface of a single crystal silicon substrate over a buried non-oxideisolation region formed in said substrate beneath said substrate surfaceand spaced from said substrate surface, said isolation region comprisingan implant-damaged region of said silicon substrate containing siliconatoms enmeshed with implanted noble gas atoms selected from the groupconsisting of neon, implanted in said substrate at a dosage level of atleast about 6×10¹⁵ neon atoms/cm² and at an energy level ranging fromabout 0.5 MeV to about 1.25 MeV; argon, implanted in said substrate at adosage level of least about 2×10¹⁵ argon atoms/cm² and at an energylevel ranging from about 1.5 MeV to about 2.5 MeV; and krypton,implanted in said substrate at a dosage level of at least about 6×10¹⁴krypton atoms/cm² and at an energy level ranging from about 3 MeV toabout 5 MeV; to inhibit recrystallization of said implant-damaged regionof said silicon substrate containing said implanted atoms of said noblegas, upon subsequent annealing of said silicon substrate.
 3. Thestructure of claim 2 which further includes oxide isolation locatedlaterally adjacent said integrated circuit structure in said substrateand extending from said surface of said substrate down to said isolationregion comprising said noble gas atoms enmeshed with said semiconductoratoms in said semiconductor substrate beneath said integrated circuitstructure.
 4. An integrated circuit structure in a single crystalsilicon substrate electrically isolated from other portions of saidsilicon substrate by a buried non-oxide isolation region formed in saidsilicon substrate beneath said integrated circuit structure, saidisolation region comprising an implant-damaged region of said siliconsubstrate containing implanted atoms of a noble gas implanted to apreselected depth in said silicon substrate beneath said substratesurface and beneath said integrated circuit structure, and at an implantdosage level sufficient to inhibit recrystallization of saidimplant-damaged region of said silicon substrate containing saidimplanted atoms of said noble gas during subsequent annealing, saidimplanted noble gas atoms enmeshed in said isolation region with siliconatoms from said single crystal silicon substrate.
 5. The structure ofclaim 4 wherein said isolation region comprises noble gas atoms selectedfrom the group consisting of neon, argon, krypton, and xenon atomsimplanted into said substrate and then annealed to form said isolationregion beneath said integrated circuit structure.
 6. The structure ofclaim 5 wherein said noble gas atoms comprise neon atoms implanted insaid substrate at a dosage level of at least about 6×10¹⁵ neon atoms/cm²and at an energy level ranging from about 0.5 MeV to about 1.25 MeV. 7.The structure of claim 5 wherein said noble gas atoms comprise argonatoms implanted in said substrate at a dosage level of least about2×10¹⁵ argon atoms/cm² and at an energy level ranging from about 1.5 MeVto about 2.5 MeV.
 8. The structure of claim 5 wherein said noble gasatoms comprise krypton atoms implanted in said substrate at a dosagelevel of at least about 6×10¹⁴ krypton atoms/cm² and at an energy levelranging from about 3 MeV to about 5 MeV.
 9. The structure of claim 5wherein dielectric isolation located laterally adjacent said integratedcircuit structure in said substrate extends down from the surface ofsaid substrate to said isolation region.
 10. The structure of claim 9wherein said dielectric isolation laterally surrounds said integratedcircuit structure in said semiconductor substrate.
 11. The structure ofclaim 9 wherein said dielectric isolation comprises field oxide formedon said substrate.
 12. The structure of claim 9 wherein said dielectricisolation comprises a dielectric material formed in a trench in saidsubstrate.